The present invention relates to FM radar systems, and in particular, to FM radar systems for detecting a direction, range, and relative speed to a target.
A radar system, which is mounted on a vehicle such as an automobile and used in conjunction with an alarm system to detect and warn of potential frontal and rearend collisions, must have a minimum range of several decimeters. For this reason, it has been recognized that for use in collision warning systems, frequency modulated-continuous wave (FM-CW) radar systems are preferred over conventional pulse radar systems. Examples of FM-CW radar systems may be found in U.S. Pat. No. 4,306,236 and Japanese Patent No. HEI 1-28915. However, the FM-CW radar systems of the prior art provide only a means for detecting a range and a relative speed to a target. These systems do not provide a means for detecting a direction to a target.
The present invention applicaton has filed a Japanese patent application, HEI 2-121366, which discloses an FM radar system capable of detecting a direction to a target in addition to a range and relative speed to the target. This Japanese application, filed Feb. 15, 1991, has not yet been laid open to public inspection.
According to the FM radar system described in the prior Japanese application, because the longest range to be detected is about several hundred meters at most, beams in the frequency range of about 60 Ghz, which are rapidly attenuated upon propagation, are radiated to avoid interference from microwave transmission systems already in existence.
To detect a direction to a target in addition to a range and relative speed to the target, a pair of antennas, each having a main lobe of the same directivity, are set to radiate beams in slightly different directions respectively. Further, a ratio of amplitudes of beat signals produced by mixing radiated, reflected, and received FM signals and non-radiated reference FM signals are detected to indicate the direction to the target.
In the disclosed FM radar system, frequencies of the beat signals are decreased as the range to a target decreases, thus causing an increase of disturbance by 1/f noise generated in the mixers. As a result, it becomes more difficult for the system to detect the frequencies and the amplitudes of the beat signals as the range to a target becomes shorter. For example, if a range to be detected is as short as one meter, the propagation delay time will be about 7 ns. In the case where the period in which frequency is increased or decreased linearly is 100 .mu.s, and a maximum change of frequency in the period is 400 Mhz, the frequencies of the beat signals will be as low as about 30 Khz. As a result, the beat signals output from the mixers will be strongly disturbed by 1/f noise. This problem is especially serious where FM radar systems are used over a very short range to detect and avoid collisions or contact between automobiles.
To solve this problem, the FM radar system, according to the prior Japanese application, raises the frequencies of the beat signals in the following manner. Specifically, the FM signals to be radiated are produced by multiplying the frequencies of original FM signals by N times, and the FM signals to be used as reference signals are produced by multiplying the frequencies of the same original FM signals by (N-1) times. However, this frequency increase results in another problem in that the modified FM radar system becomes more complicated and expensive.